xds: Make locality ID string representation consistent with A78

[arjan-bal]

Internal issue: b/397462082

The format of the locality ID used for reporting metrics is specified here: https://github.com/grpc/proposal/blob/master/A78-grpc-metrics-wrr-pf-xds.md#optional-xds-locality-label

gRPC Go uses JSON which is inconsistent with other gRPC implementations. This PR fixes this.

RELEASE NOTES:

• xds: Locality ID metric label is changed to make it consistent with gRFC A78.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.23%. Comparing base (f0676ea) to head (1ea4f19).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8256      +/-   ##
==========================================
+ Coverage   82.01%   82.23%   +0.22%     
==========================================
  Files         412      412              
  Lines       40488    40477      -11     
==========================================
+ Hits        33206    33287      +81     
+ Misses       5900     5834      -66     
+ Partials     1382     1356      -26     

Files with missing lines	Coverage Δ
xds/internal/balancer/clusterimpl/picker.go	100.00% <100.00%> (+3.00%)	⬆️
...internal/balancer/clusterresolver/configbuilder.go	92.85% <100.00%> (+1.62%)	⬆️
xds/internal/balancer/wrrlocality/balancer.go	68.13% <100.00%> (+2.86%)	⬆️
xds/internal/internal.go	76.00% <100.00%> (+7.03%)	⬆️
...ds/internal/xdsclient/xdsresource/unmarshal_eds.go	95.13% <100.00%> (ø)

... and 30 files with indirect coverage changes

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[dfawley]

Please delete assertString since it is unused now.

[arjan-bal]

Removed.